Developing and application methods of ceramic coating



United States Patent 3,399,078 DEVELOPING AND APPLICATION METHODS OFCERAMIC COATING Carlo A. M. Bang, Yokohama, Japan, assignor to SeiichiInouye, Yokohama, Japan No Drawing. Continuation-impart of applicationSer. No.

162,910, Dec. 28, 1961. This application June 2, 1965,

Ser. No. 461,229

7 Claims. (Cl. 117-129) ABSTRACT OF THE DISCLOSURE The quality ofceramic coatings for application to ferrous metal can be considerablyimproved by employing stated amounts of alkali metal borate, boric acid,a thermally alterable siliceous compound, plastic clay and an adhesive.The siliceous compound is one that either melts or dissolves orsometimes both at least by a temperature of about 1300 C. The componentsof the composition are mixed according to standard techniques andapplied to the surface of ferrous metal by processes understood in theart. Essentially, the coating is applied to the surface of the metal,dried and baked to a temperature of about 700 C. to 1300 C., preferablyat 850 C. to 1100 C. The resulting coating has enhanced hardness anddurability.

This application is a continuation-in-part of application Serial No.162,910, filed December 28, 1961 now abandoned.

The present invention concerns a single layer ceramic coating forapplication to ferrous metal surfaces such as mild steel, stainlesssteel, alloy steel, cast iron, nodular cast iron, and the like. Itfurther deals with a method for forming a single layer ceramic coatingfor application to ferrous metal surfaces thereby providing a protectivecoating film resistant to heat, oxidation, corrosion, acids, percussionand the like. It also concerns the coated ferrous metal articles.

Various processes have heretofore been proposed for applying enamels orheat-resisting enamels. Traditional enamels are usually baked on metalsurfaces using specially prepared frits for a ground coat followed by acover coat thereon, thus involving a two-stage finishing process. Ifthis were to be accomplished by a one-stage process involving a singlelayer of coating film, a saving in materials and labor would result.Unfortunately all the methods tried hitherto have shown a single-layerfilm to be poor in adhesion and generally impractical. It has heretoforebeen commonly acknowledged that enamel coatings could not besuccessfully applied as a singlelayer film. In order to form asingle-layer film, the surfaces are usually plated with metals such asnickel or zinc, or the ceramic coating is applied over a ground coatobtained by pretreatment of the metal such as with phosphates. In such amethod, special techniques and equipment are required for preparing theground coat and hence, not only is there no economic advantage butsometimes use of such a process is harmful to the quality of the groundcoat film formed on the product. Moreover, the materials which are to beenameled, such as low carbon steels and cast iron with relatively lowcombined carbon and gas content, have to be specially pre- 3,399,078Patented Aug. 27, 1968 "ice pared in order to render them suitable forenamel coating.

An object of the present invention is to provide a single-layer ceramiccoating needing neither special installation nor technique, andinvolving no surface preparation or ground coat. The coating and timerequired for forming the coating may be as little as half those requiredfor a two-layer treatment, with resulting economical advantages. Thecoatings may be applied to a wide range of ferrous materials, as statedheretofore to give a product that can withstand flexing, bending,forming, swag-ing or drawing without dislodging the ceramic coating.

The ceramic coating of the present invention comprises on a weight basis1-42%, preferably 10-35%, of an alkali metal borate, 1-10%, preferably5-8% of boric acid, 7 to 60%, preferably 10 to 40% of a thermallyalterable siliceous compound, 0.5 to 35%, preferably 4 to 30% of plasticclay, and 0.1 to 10%, preferably 2 to 8% of adhesives.

There may also be employed, if desired for special effects, 1 to 25% oftitanium oxide or chromium oxide or mixtures thereof, employed in afinely comminuted condition. Other auxiliary material known to theceramic art may be employed, if desired.

The alkali metal borate employed may be any of the several known boratesof sodium, potassium, lithium or mixtures thereof. The sodium boratesare preferred and may be typified as follows:

Sodium metaborate Na B O 4H20, NaH BO Sodium orthoborate (Na BO Sodiumpyroborate (Na B O Sodium triborate (NaB O Disodium octaborate (Na B OSodium pentaborate (NaB O Boric acid may be employed as such or in theform of its anhydride. The two embodiments are equivalent for thepresent purposes.

The thermally alterable siliceous compound includes the acid forms suchas metasilicic acid and the alkali metal and alkaline earth metal saltforms such as sodium silicate, potassium, orthosilicate, calciummetasilicate, barium metasilicate, magnesium orthosilicate, and thelike. The siliceous compounds contemplated are those that melt ordecompose or both at least by about 1300 C., preferably between about700 C. and 1300 C.

The plastic clay to be used is that typified by the ball clays known bythose skilled in the ceramic arts.

An adhesive or binding agent of either the organic or inorganic type isemployed. Typically such an adhesive includes dextrin, starch, gumarabic, gum tragacanth, gum acacia, and the like. Dextrin is thepreferred adhesive.

By rigid adherence to the above defined components and proportions, onecan consistently achieve the valuable aspects of the present invention.

In order to apply conveniently the ceramic coating of this invention oneemploys sufiicient water to make a solution of the soluble componentsand a slurry of suitable consistency of the remaining components. Theoverall consistency will frequently be that of paint as the ceramiccoating is ready for application.

The present invention may be more fully understood from the followingillustrative examples in which parts by weight are used throughout.

EXAMPLE 1 Sodium metaborate 300-800 Boric acid 100-150 Bariummetasilicate 50-100 Calcium metasilicate 100-300 Plastic clay 200-600Adhesive (dextrin) 50-200 Titanium oxide 100-400 Water 1000-2000 Thisformation is applied and allowed to dry at ambient temperature. Bakingis effected at 958-1050 C.

EXAMPLE 2 Sodium metaborate 300-800 Boric acid 100-200 Silicic acid(anhydrous) 1000-1100 Plastic clay 20-150 Adhesive (gum arabic) 10-50Sodium carbonate (anhydrous) -80 Water 1000-1500 This formation isapplied and dried at room temperature. Baking temperature is 980-ll00 C.

EXAMPLE 3 Borax 100-300 Boric acid 200-300 Crystal glass 1500-2000Plastic clay 100-300 Adhesive (gum tragacanth) 5-50 Sodium carbonate(anhydrous) 100-180 Water 1500-2000 This formulation is applied anddried at room temperature. Baking follows at 700-1100 C.

The material forming these ceramic coating materials may be mixeddirectly and applied to the entire surface, properly prepared such as bysand-blasting. They are then dried and baked. It is a characteristic ofthis invention that immediately after the drying of the coating, thebaking process may be carried out. Of course, it is also possible tofirst form a frit as in the method more usually employed for enameling.

Frit 1 (parts by weight):

Crystal :glass 1500-2000 Boric acid 150-300 Sodium carbonate (anhydrous)200-450 The above components are fused at 1300 C. and after heating for2.5-3 hours, quenched in water. After cooling, the product is crushed tomake a frit.

Slip (parts by weight):

Frit 70-85 Plastic clay 5-10 Chromium oxide -20 Water 45-60 The driedand finely divided frit is mixed in a porcelain ball mill with the othercomponents, adding adequate Water. The mixture is kneaded for 4 hours tomake up approximately 4 litres. A 50-cc. portion of the slip thusobtained when screened through a sieve of 200 mesh, should leave lessthan 0.4 gram of residue. The particle size should lie between 1 and80g, with an average particle size of 8 to 10 The slip is then applied,adding appropriate water, and is baked at a temperature of 900 to 1030C.

The conventional enamel films tend to be liable to such defects aspartial cracking or peeling and it is usual to repair such parts fromwhich the film has been removed,

. 4 a by recoating on the undercoat and baking the entire portion againin an oven.

The ceramic coating according to this invention requires no special basetreatment such as metal-plating and it can be applied as a repaircoating when ordinary enamels have been used, putting the part in anoven for reproducing perfect form again. Moreover such repairs can becompleted without the use of ovens, but by a partial application oftorches for heating. In such cases any of the above-describedsingle-layer ceramic coatings may be employed.

The ceramic coatings of this invention are simple and economical inapplication. With any of them, a single treatment of the materialsinvolved will result in an excellent single layer of the coating film.The method of application is as follows:

(1) Preparation of the metal surfaces The metal surface to be coatedshould be cleaned and freed from scale, oil, grease, and foreign matter,should be uncontaminated, and should preferably be appropriatelyroughened. Experience shows that the use of sand-blasting isadvantageous, for which the following are recommended;

Abrasives to be used (sand), mesh 40-50 Air pressure to be used, p.s.i.g60-80 Materials such as steel grits should not be used. Other treatment,for example metal plating and phosphate treatment, is not necessary.

(2) Application Either the direct application method or the methodsusing the frit can be carried out by spraying or by immersion. Thematerials composing the coating or frit are mixed in water and appliedin appropriate specific gravity and particle size. For spraying, a 50lb. air pressure is preferably used. Application by immersion isconveniently used for such places as interiors of piping materials whichare difficult to spray. In any case, the coating film thickness ispreferably in the range of 0.025 to 0.06 mm.

(3) Drying Drying is accomplished by natural drying action at room orambient temperature. In order to avoid any cracks developing on thesurface, any rapid change in temperature should be avoided.

(4) Baking The coated metal, after being completely dried, is baked inan oven at a temperature of at least 700 C. up to about 1300 C. andabove, preferably 850 C. to 1100 C., for an appropriate time so that thefilm is fused evenly. It is then left to cool. The oven may be of anyconventional type such as electric, gas or the like, and should providea constantly maintained even distribution of the temperature for uniformheating.

In the case of repairs, satisfactory results may be obtained if thedefective part and its surroundings are heated by gas burner or torch.If considered necessary, however, a whole item can be put in an oven foruniform heating.

The result, in any case, is a single-layer ceramic coating on ferrousmaterial that has superior properties of hardness, durability andresistance to wear and attack of all kinds.

1 claim:

1. A ceramic coating for application to ferrous metal consistingessentially of on a weight basis:

Percent Alkali metal borate 4-42 Boric acid 4-10 Thermally alterablesiliceous compound 7-60 Plastic clay 0.5-35 Adhesive 0.1-10

said compound being thermally alterable at least by about 1300 C.,wherein said compound is metasilicic acid or an alkali or alkaline earthmetal silicate, orthosilicate or metasilicate.

2. A single layer ceramic coating for application to ferrous metalconsisting essentially of on a weight basis:

Percent Alkali metal borate 10-35 Boric acid 5-8 Thermally alterablesiliceous compound 10-40 Plastic clay 4-30 Adhesive 2-8 said compoundbeing thermally alterable between about 700 C. and 1300 C., wherein saidcompound is metasilicic acid or an alkali or alkaline earth metal ofsilicate, orthosilicate or metasilicate.

3. A ceramic coating according to claim 1, additionally including 1 to25% of titanium oxide or chromium oxide.

4. A method of forming a ceramic coating on ferrous metal comprisingapplying the ceramic coating of claim 1 to the surface of the metal,drying said coating, and baking said coating at about 700 C. to 1300 C.

5. A method of forming a ceramic coating on ferrous metal comprisingapplying the ceramic coating of claim 2 to the surface of the metal,drying said coating, and baking said coating at about 850 C. to 1100 C.

6. A ferrous metal article having on the surface thereof a hardprotective ceramic coating resulting from the application thereto andthe drying and baking thereon the ceramic coating of claini 1.

7. A ferrous metal article having on the surface thereof a hardprotective ceramic coating resulting from the application thereto andthe drying and baking thereon the ceramic coating of claim 2.

References Cited UNITED STATES PATENTS 2,658,834 11/1953 Rex 106-482,785,091 3/1957 Rex 117-129 2,858,235 10/19'58 Rex 1'17129' X 3,255,0316/1966 Lodge et al 117129 X RALPH S. KENDALL, Primary Examiner.

